Node.js now installs `devDependencies` by default

Change effective on 01 March 2018

Many users getting started with Node.js on Heroku run a
build step with webpack, ng-cli, or another build tool
only to find that it’s in their devDependencies and
therefore not installed by default on Heroku.

We’ve updated the default behavior to install all
dependencies listed in package.json during build, and to
strip out the developer dependencies before the app is
deployed.